A TOF is an instrument for qualitative and/or quantitative chemical and biological analysis. There is an increasing need for mass analysis of fast processes, which, in part, arises from the popularity of fast multi-dimensional separation techniques such as Gas Chromatography TOF (“GC-TOF”), Mobility-TOF, Electron Monochromator TOF (“EM-TOF”), and other similar techniques. In these methods, the TOF serves as a mass monitor scanning the elution of the analyte of the prior separation methods.
There are numerous other fields of application involving the investigation of fast kinetic processes. Two examples are the chemical processes during gas discharges, and photon or radio frequency induced chemical and plasma ion etching. In the case of gas discharges, one may monitor the time evolution of products before, during, and after the abrupt interruption of a continuous gas discharge or during and after the pulsed initiation of the discharge. An analogous monitoring of the chemical processes in a plasma etching chamber may be performed. The time profile of chemical products released from a surface into a plasma can be determined either during and after the irradiation with laser pulses or before, during, and after the application of a voltage that induces etching (e.g., RF plasma processing). A third such example is the time evolution of ions either directly desorbed from a surface by energetic beams of X-ray, laser photons, electrons, or ions. In addition, when the ions are desorbed from a surface, there is usually a more predominant co-desorption of non-ionized neutral elements and molecules whose time evolution can be monitored by first post-ionizing neutral species that have been desorbed and then measuring mass separated time evolution of the ions by mass spectrometry. Yet a fourth area of use is the monitoring of the time evolution of neutral elements or molecules reflected after a molecular beam is impinged on a surface. The importance of such studies ranges from fundamental studies of molecular dynamics at surfaces to the practical application of molecular beam epitaxy to grow single crystalline semiconductor devices. A further application for fast analysis is the online analysis of aerosol particles, where the aerosol particles are sorted according to their size in time, and where the aerosols must be analyzed.
In all such studies, the time evolution of ion signals that have been mass resolved in a mass spectrometer is crucial. TOF instruments have become the instrument of choice for broad range mass analysis of fast processes.
TOF instruments typically operate in a semi-continuous repetitive mode. In each cycle of a typical instrument, ions are first generated and extracted from an ion source (which can be either continuous or pulsed) and then focused into a parallel beam of ions. This parallel beam is then injected into an extractor section comprising a parallel plate and grid. The ions are allowed to drift into this extractor section for some length of time, typically 5 μs. The ions in the extractor section are then extracted by a high voltage pulse into a drift section followed by reflection by an ion mirror, after which the ions spend additional time in the drift region on their flight to a detector. The time-of-flight of the ions from extraction to detection is recorded and used to identify their mass. Typical times-of-flight of the largest ions of interest are in the range of 10 μs to 200 μs. Hence, the extraction frequencies are usually in the range of 5 kHz to 100 kHz. If an extraction frequency of 50 kHz is used, the TOF is acquiring a full mass spectrum every 20 μs. The extraction frequency is often the fastest time scale for process monitoring. For example, monitoring a process with a TOF operating at 50 kHz extraction frequency allows for process monitoring at 20 μs time resolution. However, with special techniques disclosed in PCT application PCT/US02/16341 (Gonin et al., “A Time-Of-Flight Mass Spectrometer for Monitoring of Fast Processes”), it is possible to reduce the time resolution to one tenth or better of the extraction frequency.
Each of these fast process monitoring TOFs uses a data acquisition system based on a time-to-digital converter (TDC). Acquisition systems based on analog-to-digital converters (ADC) produce more data than can be processed by the data storage and evaluation computer. For example, a 2 GHz 8 bit ADC produces 2000 MBytes/s, which is beyond what a PCI card can transfer to a PC bus. Therefore ADC systems are used in only two cases: (1) for very short processes that must be monitored, such as for example in MALDI TOF where a LASER produces ions for a single TOF extraction, or (2) for rather slow processes that have to be monitored, where several TOF extractions could be accumulated in a fast memory internal to the ADC acquisition system, and where this memory is then periodically transferred to the PC.
In the cases where many consecutive TOF extractions have to be recorded individually (with no accumulation), the TDC technique is used. TDCs, however, have a limited dynamic range, producing one measurement per mass peak for each extraction, making it difficult to record single TOF extractions with mass peaks covering a large dynamic range (e.g., very faint mass peaks with less than one ion per extraction, and, in the same extraction, abundant mass peaks with many hundreds of ions per extraction are present).
Thus, TOFs with more effective data acquisition methods and corresponding apparatuses for monitoring fast ion processes that allow for continuous extraction monitoring with high dynamic range are needed.